A top UN rights envoy in South Sudan said child soldiers are fighting in the more than month-long conflict, with mass killings reported to have taken place.
"The reports that we have come across involve mass killings, extrajudicial killings, widespread destruction and looting, and child soldier recruitment," the UN's Assistant Secretary-General for Human Rights Ivan Simonovic told reporters.
The UN children's agency Unicef added they too had "credible reports that children are participating in the conflict", but gave no details on possible numbers.
The UN has accused forces of President Salva Kiir and former vice president Riek Machar of carrying out atrocities in the conflict that started on December 15.
"We have heard that quite a number of child soldiers are being recruited in the so-called White Army," Simonovic added, referring to an ethnic Nuer militia force that has rampaged across the eastern state of Jonglei, fighting alongside mutinous soldiers loyal to Machar in the rebel-held town of Bor.
Aid groups say up to 10,000 people have been killed in the battles and UN leader Ban Ki-moon has warned that both sides will be held "accountable".
Simonovic said it was crucial there is accountability for any human rights abuses committed.
"We shall in the coming weeks be revealing reports on preliminary findings," he added.
Meanwhile the number of South Sudanese fleeing their violence-wracked nation for severely overcrowded refugee camps in neighbouring countries could exceed 100,000 by the end of the month, the UN said Friday.
More than 86,000 South Sudanese have already flooded across the country's borders since the brutal conflict erupted.
In addition to the tens of thousands fleeing across borders, some 468,000 South Sudanese have been displaced inside the world's newest country, according to the UN's humanitarian agency.